Let me see the comparison
Trying to use Newegg, ZipZoomfly, and MonarchPC
X1900 XTX 489US vs 7900 GTX 519US,
7900 GTX wins Power Consumption decently 30W - 40W, Quieter HSF, Less Costly to Produce
X1900 XTX wins in features such as HDR + AA, Oblivion Performance/Image Quality, B&W 2.
Overall to tough to call, I would give the X1900 XTX the edge performance wise.
X1900 XT 439US vs nothing, no competitor in this space.
Winner X1900 XT
X1800 XT 512 309US vs 7900 GT 349US
7900 GT wins Power Consumption, by a great factor, 40W - 50W diff. Less costly to produce, less RAM and lower grade of 1.4ns vs 1.2ns.
X1800 XT typically wins performance wise due to better memory bandwidth management
7900 GT 349US vs X1800 XT 256 279US
7900 GT will still win the power consumption by not as much likely, Still more expensive to make then 7900 GT due to the die size and memory grade.
Unkown how performance is affected, ATI is more affected by the transistion to 512MB from 256MB of video memory. Not enough information to make a determination.
X1800 GTO 229US vs 7600 GT 189US
7600 GT wins Power Consumption, by 15W - 20W, way less costly to make due to 128Bit PCB and 125 mm^2 die size vs 256Bit PCB and 288mm^2 die. Much more compact a card as well. The 7600 GT is also a decent bit cheaper as well.
I would give the win to 7600 GT here.
6800 GS 169US vs X1600 XT 149US
(Obsolete comparison 6800 GS No longer in Stock)
Nvidia has worse costs here 210mm^2 die vs 149mm^2, as well as the need to use a 256Bit PCB, though no Low-k dielectric offsets this somewhat. It has better performance then the X1600 XT thankfully. It has higher power consumption of 12W - 15W which isn't too bad though.
X1600 XT 149US vs 7600GS 129US
Nivida has better costs here on the die and lower cost on the memory, performance comparison varies, some games 7600GS does better, some the X1600 XT does better.
7600 GS has a mild edge here.
6600 GT 128 114US vs X1600 Pro 256 109US
Pretty tight, they both perform very similarily, as well as having comparable die sizes, the 6600 GT has a more expensive memory type though only half as much. I would say this comparison is pretty even, though the X1600 is more feature rich here.
6600 DDR2 94US vs X1300 Pro 89US
The 6600 outperforms the X1300 however at a greater cost to the die size, both have equal cost on the memory side though. The X1300 is more feature rich in this category. The 6600 DDR2 has a tiny bit better power consumption thanks to low clocks.
Anything below this and it starts to get pretty tight.....
No I will probably have to make addendums to this post as it's not complete.